Types of Flu

There are three flu types -- A, B, and C. Type A influenza is the most frightening and is believed to be responsible for the global outbreaks of 1918, 1957, and 1968. The flu virus is further categorized according to where the strain was first found, a lab identification number, the year of discovery, and the type of HA and NA (surface proteins) they possess.

Types of Flu: An Overview

The first flu virus was identified in the 1930s. Since then, scientists have classified three types of flu viruses based upon their protein composition. The types of flu virus include types A, B, and C.

Type A viruses are found in many kinds of animals, including:

Ducks

Chickens

Pigs

Whales

Humans.

Type B virus widely circulates in humans. Type C has been found in humans, pigs, and dogs. Type C causes mild respiratory infections, but does not spark epidemics.

Type A influenza is the most frightening of the three flu types. It is believed to be responsible for the global outbreaks of 1918, 1957, and 1968 (see Flu Outbreaks). Type A flu viruses are subdivided into groups based on two surface proteins, HA and NA. Scientists have characterized 16 HA subtypes and 9 NA subtypes of the flu virus.

Flu Subtypes

Type A subtypes are classified by a naming system that includes:

The place the strain was first found

A lab identification number

The year of discovery

In parentheses, the type of HA and NA it possesses.

For example, A/Hong Kong/156/97 (H5N1). If the virus infects non-humans, the host species is included before the geographical site, as in A/Chicken/Hong Kong/G9/97 (H9N2). There are no type B or C subtypes of the flu virus.

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